Resin acids in pulp and paper mills wastewater are potentially partitioned in the solids in post-primary clarification due to higher hydrophobicity with log Kow ∼1.74-5.80. They are known to adversely affect anaerobic digestion (AD) process, although the effect has not been quantified deterministically in control studies. The objective of the present work was to determine the effect of untreated and ozonated spiked resin acids on AD of primary sludge. Batch adsorption tests were conducted to determine the solid-liquid partition coefficient (Kd) of resin acids on the primary sludge. Higher Kd was obtained at pH 4; however, it was decreased by 78-98% at pH 8. Thereafter, batch AD of model resin acids in primary sludge using food to microorganism ratio (S0/X) of 0.5gtCOD/gVSSindicated only 15-20% removal of resin acids in the liquid phase anaerobically. While, ozonation in pure water using 0.74-1.48 mg O3/mg tCOD showed >90% reduction of the test resin acids, an ozone dose of 0.52 mg O3/mg tCOD reduced 50-70% spiked resin acids' load to the digester. However, no further removal of resin acids occurred during AD over 30 days. About 42% reduction in methane production compared to the control digestor occurred in the presence of 150 mg/L of resin acids. When treated with 0.52 mg O3/mg tCOD, methane production improved and was comparable to the control digestor, indicating that resin acids may not be detrimental to AD at a concentration range of 45-75 mg/L.